The present invention relates to a method of measuring mode partition characteristics of a laser diode and an apparatus thereof.
Investigations for practical use of a high speed optical transmission system utilizing laser diode (LD) is recently progressing rapidly and therefore evaluation for optical transmission system is more and more considered as an important subject from the point of view of system design, etc.
In general, an output optical signal of a laser diode includes a plurality of wavelength elements, for example, .lambda..sub.0, .lambda..sub.1, .lambda..sub.2, . . . , as illustrated in FIG. 1(b) which show the spectra of output optical signal of a laser diode at times t.sub.0, t.sub.1, t.sub.2. Each wavelength element is called longitudinal mode. In case an LD outputs the optical signals at the times t.sub.0, t.sub.1, t.sub.2 corresponding to the data having a mark as illustrated in FIG. 1(a), if the three modes .lambda..sub.0, .lambda..sub.1, .lambda..sub.2 exist, the total power P.sub.t of an optical signal is partitioned in such a manner as P.sub.0 % for .lambda..sub.0, P.sub.1 % for .lambda..sub.1, and P.sub.2 % for .lambda..sub.2 (P.sub.0 +P.sub.1 +P.sub.2 =100) (Mode partitioning) However, when considering an individual optical signal corresponding to each binary data such as the PCM data, powers P.sub.0, P.sub.1, P.sub.2 of each longitudinal mode are respectively different for each optical signal at the times t.sub.0, t.sub.1, t.sub.2, and are dispersed along a certain distribution curve around the average values of P.sub.0, P.sub.1 and P.sub.2. This optical power partition to each longitudinal mode generates the noise called a mode partition noise. This noise is caused by fluctuation of the reaching time of each longitudinal mode at the receiving side. Namely, refractive index in the optical fiber is different in accordance with the wavelengths .lambda..sub.0, .lambda..sub.1, .lambda..sub.2, . . . and therefore the reaching time at the receiving side is different for each wavelength. So the power ratio of each longitudinal mode is changed, received waveform sways, and after all data error rate increases. (When .lambda..sub.1 &lt;.lambda..sub.2, the refractive index of .lambda..sub.2 becomes larger and the reaching time becomes longer.) Therefore, it is very important for system design to know the degree of mode partition.
Particularly fluctuation of said reaching time is not negligible for high speed data transmission, for example, of 405 Mb/s, 810 Mb/s or optical transmission paths with longer repeater spacing. On the contrary, data transmission rate or repeater spacing can be limited in accordance with the mode partition. A sign for determining the mode partition character is so-called a k-value (k factor). The k-value is determined uniquely for each LD.
This k-value is defined as follows. When P.sub.T is total power, P.sub.i is optical power partitioned to the longitudinal mode of wavelength .lambda..sub.i and P.sub.i is a mean value of P.sub.i, the following relation can be obtained. EQU P.sub.i.sup.2 -P.sub.i.sup.2 =P.sub.T.sup.2 .times.(.xi.)a.sub.i (1-a.sub.i)
(P.sub.i.sup.2 -P.sub.i.sup.2); dispersion
Where, x(.xi.)=1/(1+.xi.)=K.sup.2, P.sub.i =a.sub.i P.sub.T, and .xi. is a mode partition constant
Namely, a k-value is related to dispersion of optical power partitioned to each longitudinal mode. Therefore, partition of optical power to each longitudinal mode along the partition curve defined by this k-value changes for each laser pulse.
According to introducing a k-value, a ratio of the continuous spectral element w.sub.cont (f) (mode partition noise element) and line spectral element w.sub.line (f) of the received signal is given by the following equation (1). EQU w.sub.cont (f)/w.sub.line (f)=k.sup.2 (1-a.sub.i /a.sub.i).multidot.T.multidot..DELTA.f (1)
In the above equation (1),
.circle.1 k is a k-value of LD PA0 .circle.2 a.sub.i is a relative intensity of i-th longitudinal mode in the spectrum of LD output optical signal ##EQU1## .circle.3 T is a period of fixed pulse pattern .circle.4 .DELTA.f is a band width of selective level meter for measuring spectral element
Here, W.sub.cont (f), W.sub.line (f) and a.sub.i are respectively measured by measuring instruments and a wanted k-value is obtained by substituting such values into the above equation (1). An example of practical values is as follows.
______________________________________ k-value Relative strength of longitudinal mode ______________________________________ 0.36 0.55 0.29 0.12 0.25 0.071 ______________________________________
As explained above modulation of LD is carried out with the fixed pattern in the conventional mode partition character measuring system. Namely, LD is driven using a fixed pulse pattern generator. As this fixed pulse pattern, "100100100 . . . ", for example, is used. One fixed pulse pattern is set, as explained above, based on the fact that above equation (1) can be guided easily and the line spectrum and continuous spectrum can be isolated very easily.
However, such single fixed pulse pattern is too intentional and is not practical. Therefore, here arises a problem that a k-value to be obtained is not always a believable value.